Overload protection for electric motors

ABSTRACT

Electric overload protection apparatus comprising current sensitive means, connectable in an electric circuit and operative to perform an actuating function, such as to cause interruption of the electric circuit or to set off an alarm, when current flow through the current sensitive means exceeds a predetermined rated magnitude. The current sensitive means has a delayed response dependent on the magnitude of the excess current. Setting means is provided for changing the rating of the current sensitive means in a single step from a predetermined normal value to a predetermined higher value. The current sensitive means preferably includes at least one thermal sensitive bimetallic current conductor element which is arranged to flex and release a normally inoperative trigger when current flow through the bimetallic element exceeds the rated magnitude. The trigger is biased towards an operative position in which it can perform an actuating function.

United States Patent 1 Schreckenberg et al. I 1 Jan. 2, 1973 54]OVERLOAD PROTECTION FOR 3,315,054 4/1967 Langley ..337 s2 ELECTRICMOTORS 2,674,672 4/1954 Alter ..337/49 [75] Inventors: KarlSchreckenberg, Alberton;

' Louis Klein, Johannesburg; Lorenzo gummy gz g E a Antonio Read,Boksburg, all of Repubhc of South Africa ABSTRACT [73] Assignee: FuchsElectrical Industries (Proprietary) Limited, Transvaal, Electricoverload protection apparatus comprising Repbunc f South Af i currentsensitive means, connectable in an electric circult and operative toperform an actuating function, [22] Flled: 1970 such as to causeinterruption of the electric circuit or [21] Appl. No.: 96,304 to setoff an alarm, when current flow through the current sensitive meansexceeds a predetermined rated ma nitude. The current sensitive means hasa dela ed [30] Fore'gn Apphcatmu Pnomy Data resgonse dependent on themagnitude of the exc ess Dec. 12,1969 South Africa ..69/8654 current.Setting means is provided for changing the rating of the currentsensitive means in a single step [52] U.S.Cl. ..335/176,335/45,337/82from a predetermined normal value to a predeter- [51] Int. Cl. ..H0lh71/74 mined higher value. The current sensitive means [58] Field ofSearch preferably includes at least one thermal sensitive 32 bimetalliccurrent conductor element which is arranged to flex and release anormally inoperative Reiel'fllces Cited trigger when current flowthrough the bimetallic ele- UNITED STATES PATENTS ment exceeds the ratedmagnitude. The trigger is biased towards an operative position in whichit can 2,658,126 ll/l953 Filliette ..337/82 perform an actuatingfunction. 3,309,637 3/1967 Jencks ..335/l76 2,872,548 2/1969 Christensen..337/49 2 Claims, 5 Drawing Figures l ll/I/I/I/I/ lllllllll h\ lullPATENTEDJM 2191a suwzmz FIGS OVERLOAD PROTECTION FOR ELECTRIC MOTORSThis invention relates to overload protection, particularly for electricmotors.

It is well known that the overload protection of elec tric motorspresents a severe problem in view of certain contradictory requirementsthat have to be complied with. At starting, an electric motor draws ahigh current in excess of its normal rated value, the magnitude of theovercurrent and the time during which it is drawn being dependent on thetype of motor involved and the load on the motor during starting. Wherea motor is subjected to a variable load at starting, the startingcurrent and the time during which it is drawn often varies over widelimits.

Once a motor is running, it normally draws less current than atstarting, but the current drawn may still exceed the rated value due tochanges in the load imposed on the motor. Here again the magnitude ofthe overcurrent and the time during which it is drawn can vary widelydepending on the load conditions.

In providing protection for a motor, allowance must be made for the highovercurrent that is drawn during the starting period. However, if onlythe high starting current is considered, no protection would be providedfor smaller overloads during running which, although it may safely besustained for a short period, could be detrimental for the motor ifallowed to continue for an extended period. On the other hand, ifallowance is only made for lower overcurrent conditions during running,it may not be possible to start the motor since the protection systemwould trip out when the high starting current is drawn.

It is known to applicants that delayed response protection means havinga high current rating capable of handling the high starting current, isoften provided as sole protection for a motor. This is satisfactory forthe starting period, but, gives inadequate or no protection duringrunning of the motor.

To complicate matters further, quick response protection should also beprovided in the event of short circuit conditions being experienced.

Various suggestions for the overload protection of electric motors havebeen made and these suggestions have had varying degrees of success, butas far as applicants are aware, none of these suggestions have been ablesatisfactorily to cope with all the requirements either from the pointof view of complete protection or cost.

It is accordingly an object of the present invention to provide improvedoverload protection for electric motors at a more economic level.

According to the invention electric overload protection apparatusincludes current sensitive means connectable in an electric circuit andoperative to perform an actuating function when current flow through thecurrent sensitive means exceeds a predetermined rated magnitude, thecurrent sensitive means having a delayed response dependent on themagnitude of the excess current; and setting means operative to changethe rating of the current sensitive means in a single step from apredetermined normal value to a predetermined higher value.

The delayed response current sensitive means is preferably biasedtowards a condition of normal rating.

The apparatus may include adjustment means for varying the predeterminednormal value of the current rating.

The predetermined higher value of the current rating may vary insympathy with variation of the predetermined normal value.

The delayed response current sensitive means may include thermalsensitive, current conductor means displaceable in accordance with themagnitude of current flowing therethrough.

The apparatus may include trigger means biased towards an operativeposition in which it can perform an actuating function; and controlmeans normally holding the trigger in inoperative position and arrangedto be actuated by displacement of the thermal sensitive, currentconductor means to release the trigger.

Further according to the invention the apparatus may include quickresponse current sensitive means adapted to perform an actuatingfunction when the current flowing therethrough exceeds a predeterminedvalue approaching short-circuit conditions.

The quick response current sensitive means may comprise anelectro-magnetic device.

The actuating function may cause interruption of the electric circuit.

The quick response current sensitive means and the delayed responsecurrent sensitive means may be arranged to actuate common circuitinterrupting means.

Alternatively or additionally, the actuating function may set off analarm.

A preferred embodiment of the invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view from one side, of thecompartment of one of the phases of 3-phase overload protectionapparatus according to the invention comprising quick response anddelayed response current sensitive devices.

FIG. 2 is a longitudinal sectional view from the opposite side, of thedelayed response current sensitive device of FIG. 1, showing the ratingsetting means'of the device.- 1

FIG. 3 is another longitudinal sectional view from th same side'as FIG.2, of the delayed response current sensitive device, showing overloadprotection means for the thermal sensitive means of the device.

FIG. 4 is a side view of automatic actuating means for the ratingsetting means of FIG. 2

FIG. 5 is an exploded, fragmentary perspective view from the same sideas FIGS. 2 and 3, of the tripping arrangement of the delayed responsedevice of FIGS. 1 and 2 and of an arrangement for locking the ratingsetting means against displacement.

Referring first to FIG. 1, the apparatus comprises a quick responsecurrent sensitive device A connected in series with delayed responsecurrent sensitive device B.

Quick response device A may be of any suitable design and may comprise aconventional device. The device illustrated is a Westinghouse magnetictype F8 3270 ML circuit breaker comprising external terminals 1electrically connected to stationary contact 2 which is associated withmovable contact 3 which is pivotally mounted at 4 for movement betweenthe closed position shown and a raised open position. Movable contact 3is operatively connected mechanically to operating mechanism 5 which isprovided with handle 6.

predetermined value approaching short circuit conditions.

When armature 11 is attracted towards solenoid 8 in the direction ofarrow M, mechanism 13 is operated to tilt tripping bar 14 about its axis15 in the direction of arrow N to trip the circuit breaker and openmovable contacts 3. The operation of the tripping mechanism is wellknown in the art and does not form part of the present invention.

The operation of armature 11 and the tripping mechanism has a quickresponse once solenoid 8 is energized sufficiently to attract armature11.

Referring now to FIGS. 1, 2, 3 and 5, delayed response device Bcomprises three thermal sensitive, current carrying bimetallic elements16, one bimetallic element for each phase of the apparatus.

As can best be seen from FIG. 3, the bimetallic element 16 of each phaseis connected at its lower end to a terminal 10 of device B, which, inturn, is connected to a corresponding terminal 9 of quick responsedevice A. Towards its upper end, each bimetallic element 16 is connectedby means of a rigid connector 16a to a flexible lead 17 at 18. (SeeFIGS. 1 and 3). Flexible lead 17 is connected to external terminal 19 ofdelayed response device B.

When current flows through device B, bimetallic elements 16 heat upandflex transversely in the direction of arrow 0 to an extent dependingon the magnitude of the current. Upon flexing, bimetallic elements 16push against main trip bar 20.

Main trip bar 20 is connected at opposite ends to main support members21a, 21b which are pivotally mounted intermediate their ends at 22. Anadditional pair of support members 23a, 23b are pivotally mounted attheir lower ends on main support members 21a, 21b at 24 and carrybetween them at their upper ends an additional trip bar 25.

Trigger 26 is pivotally mounted on main support member 21b at 27 and isresiliently biased by means of a spring (not shown) for rotation in thedirection of arrow P in FIG. 5, towards an operative position in whichit can perform an actuating function.

Abutment 28 on trigger 26 is adapted normally to seat on stop bar 29extending between additional support members 23a, 23b. As can be seen inFIG. 5, trigger 26 is held in an inoperative position when abutment 28is seated on stop bar 29. However, when main trip bar 20 is moved asufficient distance in the direction of arrow 0 by any one or more ofbimetallic elements 16, abutment 28 moves beyond the latch 30 of stopbar 29 and is released so that the bias can exert itself and rotatetrail 31 on trigger 26 downwardly towards its operative position in thedirection of arrow P.

Movement of trigger 26 towards its operative position may be utilized toperform any required actuating function.

In the embodiment illustrated, downward movement of tail 31 on trigger26 in the direction of arrow P, causes tail 31 to depress the end 32a oftrip lever 32 downwardly in the direction of arrow R (FIGS. 1 and 5).

As can best be seen in FIG. 1, trip lever 32 extends from delayedresponse device B to quick response device A and is secured at its end32b to trip bar 14 in such a way that trip bar 14 is pivoted in thedirection of arrow N to trip the circuit breaker of device A and openmovable contacts 3, when end 32a is depressed in the direction of arrowR.

It will be seen that common circuit breaking means is provided for thetwo devices A and B and that these two devices are adapted to actuate acommon tripping device. Delayed response device B causes tripping to beeffected mechanically without an electro-magnetic shunt trip beingrequired.

It will be appreciated that the response of bimetallic elements 16 isnot instantaneous once current flow therethrough exceeds a predeterminedmagnitude. The bimetallic elements 16 have a delayed response in wellknown manner as they beat up and flex with increased current flowtherethrough. The response is dependent on the magnitude of the excesscurrent and becomes more rapid as the magnitude of the excess currentincreases.

The amount of flexing of bimetallic elements 16 and thus the magnitudeof current flowing through elements 16, which is required before thecommon tripping device is actuated, depends on the relative positioningof abutment 28 of trigger 26 and the latch 30 of stop bar 29. Thecurrent rating at which delayed response device B will actuate thetripping device can thus be set by adjusting the position of stop bar 29relative to abutment 28 so as to adjust the extent to which they overlapin the inoperative position of trigger 26 shown in FIG. 5.

As shown in FIG. 5, additional trip bar 25, additional support members23a, 23b and stop bar 29 are biased towards main trip bar 20 by means ofspring 33 between main and additional support members 21a, 23a.

Control arm 34 is fast with stop bar 29 and with additional supportmember 23a and is adapted to bear with its free end against stop member35. The position of stop member 35 is adjustable so that the position ofstop bar 29 relative to abutment 28 is adjustable, thereby to permitadjustment of the current rating at which the delayed response devicewill trip the circuit breaker.

Stop member 35 is secured to adjustment lever 36 which is pivotallymounted at 37 on base 38 and is biased by spring 63 in the direction ofarrow S about 37 towards control arm 34. The movement of adjustmentlever 36 in the direction of arrow S is limited by base 38 on whichadjustment lever 36 is mounted. The normal position of stop member 35when support 36 abuts base 38, can be adjusted by means of calibrationscrew 61 which permits the spacing between adjustment lever 36 Y and thetail 35a of stop member 35 to be adjusted. Tail 35a of stop member 35 isin the nature of a leaf spring which biases itself towards adjustmentlever 36.

By adjustment of calibration screw 61, the normal value of the currentrating of delay response device B is obtained. Calibration screw 61 maythen be locked in position such as by detachably securing it to tail 35aof stop member 35 by means of solder.

As can best be seen from FIGS. 2 and 5, the rating of the delayedresponse device B can be changed in a single step from its normal valueto a predetermined higher value, by means of displacement member 39which can be moved downwardly in the direction of arrow T by means ofpush button 40.

Normally, displacement member 39 is biased upwardly by means of spring62 (FIG. 2) towards the inoperative position shown in which tail 41 onadjustment lever 36 bears against cam profile 39a of displacement member39. In this position, the rating of delayed response device B is at itsnormal value.

When a higher current rating is required from delay response device B,push button 40 is depressed to move displacement member 39 downwardly inthe direction of arrow T so that it presses against the inclined end oftail 41 on adjustment lever 36 and pivots adjustment lever 36 and stopmember 35 about 37 in a direction opposite to that shown by arrow S.This causes control arm 34 to move in the direction of arrow U, therebyto cause latch 30 of stop bar 29 to move further underneath abutment 28of trigger 26 in the direction of arrow X. It will be appreciated thatincreased flexing of bimetallic elements 16 is required to move abutment28 off stop bar 29 and cause tripping of the circuit breaker. Theapparatus can therefore carry an increased current without tripping ofthe circuit breaker.

In order to permit adjustment of the normal current rating of delayresponse device B as obtained by the setting of calibration screw 61,displacement member 39 is providedvwith curved cam profile 39a (FIG. 5)and is rotatably mounted with push button 40 in mounting bracket 60. Bysimply rotating push button 40, difierent zones of cam profile 39a lyingat different distances from the axis of rotation can be located oppositeadjusnnent lever tail 41. The normal positions which displacement lever36 and stop member 35 assume under the biasing influence of spring 63when displacement member 39 is in its inoperative position, will dependon the particular zone of cam profile 39a engaging adjustment lever tail41. By varying the circumferential position of displacement member 39,the normal position of displacement lever tail 41 can be varied. Theposition which stop bar 29 assumes relative to abutment 28 of trigger 26can thus be adjusted to vary the current rating setting of delayresponse device B under normal conditions.

The circumferential position of displacement member 39 influences notonly the normal position of adjustment lever tail 4], but also theposition to which adjustment lever tail 4] is displaced by displacementmember 39, when push button 40 is depressed to change the rating in asingle step from a predetermined normal to a predetermined higher value.Rotation of push button 40 therefore varies the higher value of currentrating in sympathy with variation of the normal value.

As can be seen from FIG. 2, calibration screw 61 for setting the normalrunning current rating of device B, is located within the casing 43 ofthe device and cannot be tampered with to alter the setting, withoutopening the casing. Push button 40 is, however, easily accessible fromthe outside of the casing.

In order to avoid accidental or mischievous alteration of the normalcurrent rating, the device B may be located behind a panel door 42 asshown in FIG. 5. Push button 40 is adapted to extend through aperture 44in casing 43 and through a locking member 45 presenting a flange 46adapted to seat in external recess 47a in casing 43 and to abut againstthe inside of panel door 42 when the panel door is in position againstcasing 43 with locking member 45 passing through an aperture in paneldoor 42. Engagement of flange 46 with the surrounds of the aperture inpanel door 42, prevents withdrawal of locking member 45 through thepanel door 42.

A locking pin 47 extends from locking member 45 through aperture 48 incasing 43 into the interior of device B and is adapted to engage in oneor other of the notches 49 in the upper part of displacement member 39,depending on the circumferential setting of displacement member 39. Thisprevents rotation of displacement member 39. If the normal currentrating is to be altered, the panel door 42 and locking member 45 havefirst to be removed to permit rotation of displacement member 39.

As stated above, quick response device A is arranged to operate onlywhen the current flow through the apparatus exceeds a predeterminedvalue approaching short circuit conditions. In order to avoid damage tothe bimetallic elements 16 by high currents below the predetenninedshort-circuit value, a single turn type of solenoid 50 is connected inseries with each of the bimetallic elements 16 as shown in FIG. 3. Anarmature comprising a movable contact 51 is pivotally mounted at 52 andis connected to flexible lead 17. When the current through a bimetallicelement 16 exceeds a predetermined value, solenoid 50 attracts movablecontact 51 to engage a contact (not shown) at the lower end of thebimetallic element 16. It will be appreciated that a shunt path iscompleted through flexible lead 17 from point 18, through movablecontact 51 to the bottom of bimetallic element 16, thereby to divertcurrent flow from the bimetallic element 16.

The apparatus described above may be used for any suitable overloadprotection. It may,'for example, be connected in series with athree-phase electric motor. The rating of delayed response device B maybe adjusted by rotation of displacement member 39, to a required normalrunning value equivalent to motor full load current, say 50 amps, whichwould provide adequate protection while the motor is running. Bydepressing push button 40, a higher starting rating is obtained whichwould allow sufficient current to be drawn to permit starting of themotor under normal conditions and which would not cause tripping of thecircuit breaker during the normal starting period and under normalstarting conditions. However, delayed overload protection is stillobtained if too high a current is drawn for too long a period of time aswell as quick response protection if the current exceeds locked rotorcurrent values.

When the motor is to be started, push button 40 is depressed to increasethe rating of delayed response device B in a single step from the normalrunning value to the starting value. The rating is held at the startingvalue by holding button 40 depressed, until the motor is runningnormally, whereafter button 40 is released to permit device B to returnto a condition of normal rating under the influence of biasing springs62 and 63. In this manner, the motor is allowed to start and adequateprotection is provided both during starting and during running of themotor.

It will be appreciated that many variations in detail are possiblewithout departing from the scope of the appended claims.

For example, push button 40 for changing the current rating from thenormal value to the higher value may be actuated manually orautomatically. An automatic arrangement is illustrated in FIG. 4 where aremotely energizable solenoid 53 is provided with an armature 54 pivotedat 55 and extending over push but: ton 40. When solenoid 53 isenergized, armature 54 is attracted downwardly towards solenoid 53 todepress push button 40. When solenoid 53 is de-energized push button 40is released. The operation of solenoid 53 may be with or without a timedelay.

The tripping arrangement of FIG. 5 includes the facility that thecircuit breaker of quick response device A is tripped when an opencircuit occurs in any of the phases. When such an open circuit occurs,current flow through the relevant bimetallic element 16 is interruptedso that it straightens out and moves in the direction of arrow Y in FIG.5, thereby to displace additional trip bar 25 in the same direction.Such displacement pushes stop bar 29 which is fast with additional tripbar 25, from underneath abutment 28 of trigger 26, thereby allowing thetrigger to actuate the tripping mechanism.

Delayed response device B may be used in conjunction with any suitablecircuit breaking device other than quick response device A. Where deviceB is used with a manually operated circuit breaker without an automatictripping device such as solenoid 8, the arrangement of FIG. 3 may beadapted to provide automatic tripping on excessive overload. Thus tail51a on movable contact 51 may be adapted to engage stop bar 29 anddisplace it in the direction of arrow Y when movable contact 51 isattracted by solenoid 50 as described above. Displacement of stop bar 29causes operation of trigger 26 which can be utilized in any suitablemanner to open the circuit interrupting device.

Trigger 26 may be arranged to cause interruption of the electricalcircuit in which it is connected, to be effected mechanically withoutany additional electrically operated shunt or under voltage releasedevice in the circuit interrupting device being required.

In an alternative arrangement, movement of tail 31 of trigger 26 towardsits operative position in the direction of arrow P, may be utilized tooperate a micro-switch or any other suitable control or actuatingswitch. Operation of the control or actuating switch may be utilized tocause a circuit interrupting device to be opened electro-magnetically,such as by means of a shunt or under voltage tripping device. Thisarrangement permits delayed response device B to be located remotelyfrom the circuit interrupting device.

Additionally or alternatively, the operation of the control or actuatingswitch may be utilized to set off an alarm which may be visual oraudible. It will be aprecia ed th t movement tr' er 26 towards itsperative position may be utiiized perform any suitadisplacement of thethermal sensitivecurrent conductor means to release the trigger; andsensitivity setting means operable upon the control means andresiliently biased towards a normal position at which the sensitivity ofoperation of the control means is maintained at a normal operating valueso that the trigger is released when current flow through the thermalsensitive current conductor exceeds a predetermined normal magnitude,the sensitivity setting means being displaceable against the action ofthe bias to an overcurrent position at which the sensitivity ofoperation of the control means is decreased temporarily to a value suchthat the trigger is ',released only when an overcurrent of predeterminedmagnitude in excess of the normal magnitude flows through the thermalsensitive conductor means, the sensitivity setting means beingautomatically returnable to its normal position by the bias when notbeing displaced.

2. Electric overload protection apparatus including trigger means biasedtowards an operative position in which it can perform an actuatingfunction; stop means normally holding the trigger in inoperativeposition, the trigger and stop means being relatively displaceable torelease the trigger, the relative positioning of the stop means and thetrigger when the latter is held inoperative being variable to vary therelative displacement required to release the trigger; at least onethermal sensitive, bimetallic current conductor element connectable inan electric circuit and arranged to flex to an extent dependent on themagnitude of current flowing therethrough, flexure of the bimetallicelement acting to relatively displace the stop means and the trigger;and sensitivity setting means operative to change the relativedisplacement between the trigger and the stop means required to releasethe trigger, the sensitivity setting means being resiliently biasedtowards a normal position at which the relative positioning of thetrigger and the stop means is held in a normal condition so that thetrigger is released after a predetermined normal relative displacementcaused by current flow through the bimetallic element in excess of apredetermined normal magnitude, the sensitivity setting means beingdisplaceable against the action of its bias to an overcurrent positionat which the relative positioning of the trigger and the stop means istemporarily changed to a condition such that the trigger is onlyreleased after an increased relative displacement caused by anovercurrent of predetermined magnitude in excess of the normalmagnitude.

1. Electric overload protection apparatus comprising delayed responsethermal sensitive; current conductor means connectable in an electriccircuit and displaceable in accordance with the magnitude of currentflowing therethrough; trigger means biased towards an operative positionin which it can perform an actuating function; control means normallyholding the trigger in inoperative position and arranged to be actuatedby displacement of the thermal sensitive current conductor means torelease the trigger; and sensitivity setting means operable upon thecontrol means and resiliently biased towards a normal position at whichthe sensitivity of operation of the control means is maintained at anormal operating value so that the trigger is released when current flowthrough the thermal sensitive current conductor exceeds a predeterminednormal magnitude, the sensitivity setting means being displaceableagainst the action of the bias to an overcurrent position at which thesensitivity of operation of the control means is decreased temporarilyto a value such that the trigger is released only when an overcurrent ofpredetermined magnitude in excess of the normal magnitude flows throughthe thermal sensitive conductor means, the sensitivity setting meansbeing automatically returnable to its normal position by the bias whennot being displaced.
 2. Electric overload protection apparatus includingtrigger means biased towards an operative position in which it canperform an actuating function; stop means normally holding the triggerin inoperative position, the trigger and stop means being relativelydisplaceable to release the trigger, the relative positioning of thestop means and the trigger when the latter is held inoperative beingvariable to vary the relative displacement required to release thetrigger; at least one thermal sensitive, bimetallic current conductorelement connectable in an electric circuit and arranged to flex to anextent dependent on the magnitude of current flowing therethrough,flexure of the bimetallic element acting to relatively displace the stopmeans and the trigger; and sensitivity setting means operative to changethe relative displacement between the trigger and the stop meansrequired to release the trigger, the sensitivity setting means beingresiliently biased towards a normal position at which the relativepositioning of the trigger and the stop means is held in a normalcondition so that the trigger is released after a predetermined normalrelative displacement caused by current flow through the bimetallicelement in excess of a predetermined normal magnitude, the sensitivitysetting means being displaceable against the action of its bias to anovercurrent position at which the relative positioning of the triggerand the stop means is temporarily changed to a condition such that thetrigger is only released after an increased relative displacement causedby an overcurrent of predetermined magnitude in excess of the normalmagnitude.